Accelerator Mass Spectrometry Radiocarbon Analysis Research Articles

Biobased carbon content quantification through AMS radiocarbon analysis of liquid fuels

Increased production and use of renewable hydrocarbons has created a need for accurate quantification of the biobased carbon content of hydrocarbon fuels. The blend percentages of conventional biofuels (such as biodiesel and ethanol) can be quantified by numerous methods such as gas chromatography and infrared spectroscopy. Distinguishing the biobased content of a purely hydrocarbon fuel, however, is more difficult. In this study, a single-blind trial was conducted to assess the accuracy and precision of accelerator mass spectrometry (AMS) radiocarbon analysis of the biobased carbon content of hydrocarbon fuels. Test samples with known biobased carbon contents ranging from 0 to 10 wt% were sent to four labs for analysis by ASTM D6866-16 Method B. Blank correction of the test results was found to improve measurement accuracy through the reduction of lab-specific systematic error. Favorable precision, accuracy, and a limit of detection were determined. This study strongly supports radiocarbon analysis by AMS as an accurate and reliable method for quantifying the biobased carbon content of hydrocarbon transportation fuels.

A multidisciplinary investigation of a rock coating at Ngaut Ngaut (Devon Downs), South Australia

This paper presents the results of a multidisciplinary investigation into a dark rock coating at the Ngaut Ngaut heritage complex in South Australia (SA) using geological and botanical examination, Raman microscopy, x–ray powder diffraction, scanning electron microscopy and infrared analyses. The coating analysed contains a mixture of calcite, quartz, gypsum and weddellite. The presence of calcite and quartz can be explained by the underlying clastic fossiliferous limestone, while the most probable explanation for the origin of the gypsum is via ground water. The weddellite was likely formed from solutions derived from the reaction of calcite with oxalic acid through the intervention of surface microflora, such as algae. This article provides the first record of weddellite in any context in SA. These findings have a number of implications—one being that the oxalate mineral in the rock coating could potentially be used to conduct accelerator mass spectrometry radiocarbon analysis and thereby refine our understanding of the rock art chronology at Ngaut Ngaut. A greater understanding of the rates of accumulation may also be useful for management purposes, as the nature of the rock coating may be contributing to long–term exfoliation. Indeed, it is argued that algal colonisation of the limestone (together with other probable microfloral activities) is likely involved in the production of a film over the porous surface, leading to salt weathering

A Wet Oxidation Method for AMS Radiocarbon Analysis of Dissolved Organic Carbon in Water

We present a method for the extraction of dissolved organic carbon (DOC) from water. The method is adapted from Burr et al. (2001) using the basic steps: 1) sample filtration; 2) acidification to liberate and remove dissolved inorganic carbon (DIC); 3) evaporation of the sample to isolate salts that include trace quantities of carbon; 4) combustion of the salts; and 5) purification of the CO2. Two significant improvements have been made to the earlier method. The first is to use wet oxidation with potassium permanganate to oxidize organics in place of the combustion step and the second is the development of a reduction/oxidation purification procedure to remove sulfur and nitrogen oxides that may form during the oxidation step. Wet oxidation has a practical advantage over the previous method because it proceeds at low temperature (70 °C). The original method required quartz vessels to oxidize the salts at 900 °C. At this temperature, salts in the samples formed gases that interfered with the isolation of CO2 and the quartz vessels degraded with each combustion, affecting their structural integrity. The expensive quartz vessels could only be used for a limited number of samples, whereas Pyrex vessels used for wet oxidation are inexpensive and can be used indefinitely.The blank fraction modern carbon (f) and its mass dependence for the refined technique was determined from repeat analyses of salicylic acid produced from petrochemicals. For samples with a mass m above 0.5 mg, F = 0.0083 ± 0.0011. For samples below 0.5 mg, the blank follows a 1/m dependence as observed for other accelerator mass spectrometry (AMS) 14C measurements (Donahue et al. 1990). The reproducibility of the method is demonstrated using repeat measurements from a variety of samples, including a sample measured with the former high-temperature 900 °C combustion technique. The virtues of the wet oxidation method are that it is economical, produces a low blank, and provides good reproducibility.

A Wet Oxidation Method for AMS Radiocarbon Analysis of Dissolved Organic Carbon in Water

We present a method for the extraction of dissolved organic carbon (DOC) from water. The method is adapted from Burr et al. (2001) using the basic steps: 1) sample filtration; 2) acidification to liberate and remove dissolved inorganic carbon (DIC); 3) evaporation of the sample to isolate salts that include trace quantities of carbon; 4) combustion of the salts; and 5) purification of the CO2. Two significant improvements have been made to the earlier method. The first is to use wet oxidation with potassium permanganate to oxidize organics in place of the combustion step and the second is the development of a reduction/oxidation purification procedure to remove sulfur and nitrogen oxides that may form during the oxidation step. Wet oxidation has a practical advantage over the previous method because it proceeds at low temperature (70 °C). The original method required quartz vessels to oxidize the salts at 900 °C. At this temperature, salts in the samples formed gases that interfered with the isolation of CO2 and the quartz vessels degraded with each combustion, affecting their structural integrity. The expensive quartz vessels could only be used for a limited number of samples, whereas Pyrex vessels used for wet oxidation are inexpensive and can be used indefinitely.The blank fraction modern carbon (f) and its mass dependence for the refined technique was determined from repeat analyses of salicylic acid produced from petrochemicals. For samples with a mass m above 0.5 mg, F = 0.0083 ± 0.0011. For samples below 0.5 mg, the blank follows a 1/m dependence as observed for other accelerator mass spectrometry (AMS) 14C measurements (Donahue et al. 1990). The reproducibility of the method is demonstrated using repeat measurements from a variety of samples, including a sample measured with the former high-temperature 900 °C combustion technique. The virtues of the wet oxidation method are that it is economical, produces a low blank, and provides good reproducibility.

Holocene fires, forest stability and human occupation in south‐western Amazonia

AbstractAimThe aim of this study was to assess the effect of Holocene fire activity and drought on the mesic forests of the Upper Beni and the extent of ecotone migration in south‐western Amazonia during the mid‐Holocene. An additional goal was to address the hypothesis of Amazonia as a manufactured landscape prior to the arrival of European colonizers in the New World in ad 1492.LocationLakes Chalalán and Santa Rosa, Upper Beni, Bolivian Amazon.MethodsHolocene vegetation and fire activity were reconstructed based on pollen and microcharcoal records from sediments of lakes Chalalán and Santa Rosa. Chronologies were based on accelerator mass spectrometry radiocarbon analyses, and vegetation changes were summarized using detrended correspondence analysis. The normalized difference vegetation index was used to classify and determine the extent of the modern savanna–forest ecotone using MODIS satellite image data.ResultsMesic evergreen forests persisted in the Upper Beni throughout the Holocene. Fire was frequent during the early and mid‐Holocene and temporally consistent in both records. Ordination of pollen data showed an increase in forest change during the late Holocene when fire activity was asynchronous in the two records.Main conclusionsThe Upper Beni sites were not reached by expanding savannas during periods of major environmental change, suggesting forest resilience and a degree of ecotone stability. We associated the largest observed change in these forests with late Holocene fires that were most probably ignited by humans. We found little evidence supporting widespread pre‐1491 cultural landscapes.

Late quaternary vegetation dynamics and human impact on Alexander Selkirk Island, Chile

AbstractAim To reconstruct the history of vegetation and environments using pollen, charcoal and sediment analysis, and to identify the timing and nature of climate change and human impact on the vegetation of a remote Pacific island.Location Cerro de Los Inocentes, 1000 m above sea level, Alexander Selkirk Island (33°45′S, 80°45′W), Chile. The westernmost island of the Juan Fernandez Archipelago, south‐east Pacific Ocean.Methods A 150‐cm long sediment core comprising 87 cm dark brown peat overlying 63 cm of yellow grey clay was extracted from a shallow depression on the southern slopes of Cerro de Los Inocentes. Pollen, charcoal, sediment and accelerator mass spectrometry radiocarbon analyses were used to construct a record of vegetation change through time. Numerical analysis of multispecies data allowed the classification of fossil assemblages into distinct pollen zones.Results Pollen and spores are preserved throughout the sediment with high concentrations coinciding with the beginning of organic sediment accumulation at around 8000 14C yr BP. Prior to 8000 14C yr BP, the deposition of clays, presumably from upslope erosion, occurred in a landscape sparsely vegetated by grasses, ferns and Pernettya rigida heath, including several plants that are only found 100–200 m above the site today (Zone CI‐1). After 8000 14C yr BP, a P. rigida heath was the dominant vegetation (Zone CI‐2). A shift to a wet heath–shrubland (Zone CI‐3) occurred at 6000 14C yr BP and was followed by a transition to a treefern–shrubland mosaic accompanied by periodic burning (Zone CI‐4) after 4500 14C yr BP. The impact of human occupation is evident in Zone CI‐5 at 450 14C yr BP with the loss of forest species, increased burning and invasion of the exotic plant Rumex.Main conclusions The pollen and charcoal record provides the first evidence of vegetation changes spanning at least the last 8000 14C yr BP from the high altitude environment of Alexander Selkirk Island. Prior to 8000 14C yr BP, the altitudinal ranges of different plant species may have been suppressed by a cooler and drier climate. Increasing precipitation and temperatures at the end of the last glacial period may have mobilized exposed sediments in a sparsely vegetated upland environment, altering local drainage patterns, eventually leading to slope stabilization and deposition of organic detritus under an increasing density of heath and shrub vegetation. The subalpine heath–shrubland persisted until 4500 14C yr BP when first evidence for sustained burning is found in association with the establishment of a more open treefern–shrubland vegetation pattern. In the absence of human occupation at this time, the influence of increased climatic variability associated with more frequent El Niño‐Southern Oscillation events during the mid to late Holocene is considered one of the main driving forces behind increased vegetation disturbance during this period. The record provides evidence that island vegetation patterns have been highly dynamic over millennial to decadal time‐scales and that the flora has persisted through periods of rapid and major climate change. This changed with the discovery of the island by European explorers in the late sixteenth century and the subsequent introduction of goats and exploitation and burning of forests, which resulted in the progressive destruction of native vegetation and the invasion of introduced plants. There is evidence that reduced burning and control of the goat population within the last 50 years has resulted in marginal recovery of some high altitude native plant species.

Radiocarbon Reservoir Ages in the Mediterranean Sea and Black Sea

We measured apparent marine radiocarbon ages for the Mediterranean Sea, Black Sea, and Red Sea by accelerator mass spectrometry radiocarbon analyses of 26 modern, pre-bomb mollusk shells collected living between AD 1837 and 1950. The marine reservoir (R(t)) ages were estimated at some 390 ± 85 yr BP, 415 ± 90 yr BP and 440 ± 40 yr BP, respectively. R(t) ages in the Mediterranean Sea and Black Sea are comparable to those for the North Atlantic Ocean (<65°N), in accordance with the modern oceanic circulation pattern. The ΔR values of about 35 ± 70 yr and 75 ± 60 yr in the Mediterranean area show that the global box-diffusion carbon model, used to calculate R(t) ages, reproduces the measured marine14C R(t) ages in these oceanic areas. Nevertheless, high values of standard deviations, larger than measurement uncertainties are obtained and express decadal R(t) changes. Such large standard deviations are indeed related to a decrease of the apparent marine ages of some 220 yr from 1900 AD to 1930 AD in both the Mediterranean Sea and the western North Atlantic Ocean.

Rock Dates Thrown Into Doubt, Researcher Under Fire

The age of some striking Arizona petroglyphs is being called into question in a Technical Comment in this issue (p. 2132) challenging the validity of the technique used. The method uses accelerator mass spectrometry radiocarbon analysis to date microscopic quantities of carbon-rich organic material that are believed to become trapped beneath a thin layer of natural varnish on rocks. The researcher who pioneered the method acknowledges that his technique is flawed and produces ambiguous results but counters that faulty lab technique explains why other researchers have been unable to replicate his results. The controversy has sparked inquiries by the National Science Foundation and the university where the research was performed and is likely to cast a cloud over rock-varnish science as a whole.

Distribution of otoliths in surficial sediments of the U.S. Atlantic Continental Shelf and slope and potential for reconstructing Holocene fish stocks

We examined more than 1100 surface sediment samples from the Atlantic continental margin of the United States to determine the feasibility of using fossil fish otoliths as diagnostic tools in reconstructing paleoenvironments and latitudinal distribution of fish stocks during the Holocene. Although 63% of the 1107 samples collected were from shelf areas (<140 m), the total number of shelf‐derived otoliths represents only 0.3% of the entire sampled assemblage. The majority of otoliths occurred on the continental slope (400‐2000 m), with a maximum concentration in sediments at 500 to 600‐m water depth. Otoliths of the most commonly occurring species, Ceratoscopelus maderensis, exhibit a marked distributional boundary just south of Cape Hatteras, North Carolina (33°N), which mimics the distribution of their living counterparts. North of this boundary, C. maderensis constitutes greater than 70% of the preserved otolith assemblage, whereas more southerly regions contain no otoliths of this species. Although C. maderensis typically migrates diurnally over a depth of 300‐600 m, otoliths taken from live‐captured C. maderensis exhibit Δ14C values comparable to that of the dissolved inorganic carbon (DIC) of surface seawater in the study area. Accelerator mass spectrometry radiocarbon analyses of cooccurring otoliths and planktonic foraminifera from a sediment core collected south of Martha’s Vineyard (40°15′N 70°51′W, 265 m) demonstrate temporal concordance throughout the Holocene. Otoliths appear to be viable, underutilized paleoceanographic tools. Specimens are found in sufficient abundance to permit temporal reconstructions of the distribution of C. maderensis and potentially several other icthyospecies along the U.S. Atlantic continental margin.

New Chronometric Dates for the Puquios of Nasca, Peru

Numeric age assessments of organic material encapsulated by rock varnish present on stone lintels of two “puquios” in the Nasca region of the south coast of Peru indicate a Precolumbian age. Accelerator mass spectrometry radiocarbon analyses at the Isotrace Laboratory revealed ages of cal A. D. 560(620)650 and cal A. D. 600(640)660 (1 sigma). These calibrated measurements are considered to be appropriate in the context of the archaeological settlement pattern data for the region. As well, the written documents of the colonial period neither confirm nor deny the presence of the puquios. Three assumptions are inherent in the acceptance of these dates: (1) AMS14C dating and its application to rock varnish yield reliable results, (2) sample contamination can be recognized, and (3) determination of the age of the puquios rests upon testable available information; ex silentio arguments about the presence or absence of the puquios in the Nasca region before the Spanish conquest are inadequate for the resolution of the question.